Filter aids are divided substances which, when used in solid-liquid separations by placing them on a screen or support, either alone or as a mixture with the particles to be retained, provide or improve, by a mainly mechanical action, the efficacy of the separation.
This efficacy may be related either qualitatively to the filtrate obtained, for example its purity or its sterility, or quantitatively by the increase in the flow rate of filtrate.
The main aids used are:                diatomaceous earths obtained from the calcination of diatomites, algal fossils forming part of the plankton family and generally referred to as kieselguhr;        perlites originating from a volcanic rock, rhyolite. This crushed, ground rock is expanded more than 20 times in a calcination oven;        starch in the form, for example, of potato flour;        fly ash;        cellulose;        synthetic materials such as polymer fibers, glass beads or plastic beads, etc.        
These aids form, during filtration, a porous medium which captures the impurities to be removed and facilitates the flow of the liquid phase.
It is known that these aids can be used either as a prelayer or in alluviation.
As a prelayer, the aid is placed on the filter medium prior to filtration of the suspension.
In alluviation, the aid is mixed with the suspension, prior to filtration, generally by means of a metering pump. This suspension forms a mixed cake composed of aid and impurities. It is very common to filter a suspension with alluviation on a prelayer, in order to prepare a well-clarified initial filtrate, to protect the filter medium from the migration of fine particles which could block it and to favor the dismantling and cleaning of the filtration equipment.
One particularly advantageous sector of application of aids is that of beermaking.
European patent application EP-A-0,483,099 describes a filter aid intended to be used more particularly in the technique of alluviation in the field of brewing. This aid consists of spherical beads of particle size between 5 μm and 50 μm with an average diameter close to 20 μm. These aids are preferably used in the form of cakes whose porosity is between 0.3 and 0.5. These spherical beads are incompressible beads which are resistant to abrasion and to regeneration agents, and relatively insensitive to the effects of temperature and feed quality. These are usually made of solid or hollow glass or plastic.
U.S. Pat. No. 4,326,964 describes a process of filtration on a deep bed. According to that process, the liquid filled with impurities percolates through a filter bed, thus gradually blocking it. In order to increase the duration of the cycle and to limit the blocking of the filter at the surface, it is proposed to have a bed formed of layers of different particle sizes in which the largest particles are found at the top of the bed and stop the bulkiest impurities, and the finest particles are found at the bottom of the filter, in order to trap the smallest impurities. When the bed is blocked, it is envisaged to carry out a cleaning operation which is performed by countercurrent washing at a flow rate such that a fluidization phenomenon occurs. This action causes mixing between the various layers of different particle sizes. To prevent this homogenization of the layers of different particle sizes, it is proposed to use materials whose specific masses are suitably chosen to cause segregation. In particular, the addition of hollow silica beads in cement is one means proposed to achieve this objective.
It would be advantageous to provide a filter aid intended to be used in alluviation techniques that is of improved behavior when compared with those described in the prior art, and more particularly that described in document EP-A-483,099.
It would also be advantageous to provide a filter aid that is particularly suitable for use in beermaking, and more particularly for the technique of alluviation intended for the clarification of beer. Furthermore, it would be advantageous to provide a filter aid that produced a filter cake having a reduced specific resistance, a reduced increase in pressure drop across the filter cake, and a reduced rate of filter cake growth.